Strict clusterIssuer also for cyberchef and urlaubsverwaltung

Per the maintainer decision (option a: ingress.clusterIssuer strictly
required in ALL charts) the two remaining defaults are removed as well:

- cyberchef: drop the letsencrypt default; bump raised from minor to
  major (6.0.0+up11.4.0) as this is a breaking values change.
- urlaubsverwaltung: drop the letsencrypt-issuer default; major bump
  to 6.0.0+up6.7.0.
- CI test-values for both charts now set clusterIssuer explicitly.

A repo-wide grep confirms no chart carries a clusterIssuer default
anymore.

cyberchef/Chart.yaml

@@ -5,7 +5,7 @@ icon: https://raw.githubusercontent.com/gchq/CyberChef/master/src/web/static/ima
 
 type: application
 
-version: 5.2.0+up11.4.0
+version: 6.0.0+up11.4.0
 appVersion: "11.4.0"
 
 maintainers:

cyberchef/values.yaml

@@ -60,7 +60,7 @@ cyberchef:
 
 ingress:
   enabled: true
-  clusterIssuer: letsencrypt
+  clusterIssuer: null
   domain: null
   className: nginx
   # Extra annotations for the Ingress; merged with the cert-manager